Two-stage intensifiers, sometimes called boosters, are common in the prior art as disclosed, for example, in U.S. Pat. Nos. 3,787,147; 3,761,204; 3,279,381; and 3,625,006. In all of the aforesaid patented structures, the fluid displacement pistons are reciprocated by supplying pressure to both sides thereof which unnecessarily complicates their construction. The device of this invention is designed to eliminate such complexity by employing spring means for returning the fluid displacement pistons to their initial positions with the advantage that the residual low pressure is removed from the reservoir.